Chandra's image of a puzzling
X-ray source in the globular star cluster M15 shows
that it is not one neutron star binary system, but two
neutron star binary systems that appear so close
together (2.7 seconds of arc) that they were
indistinguishable with previous X-ray
telescopes.

In the 1970's astronomers discovered one neutron star
binary system in M15, called 4U2127, with the Uhuru
X-ray satellite. Subsequent data from X-ray telescopes
indicated that the neutron star itself was not directly
visible in X-ray light because it was hidden by an
accretion disk of hot matter swirling from a companion
star onto the neutron star. This picture was put into
doubt when the Japanese Ginga X-ray satellite saw
luminous X-ray bursts from the region in 1990. The
length of the burst and other light characteristics
implied that the surface of the neutron star was
directly visible, in contradiction with earlier
observations.

Chandra observations solved this mystery. The source
could exhibit two contradictory modes of behavior
because 4U2127 is not one source, but two: one whose
neutron star is hidden by an accretion disk (on the
left in the image), and one (right) where occasional
X-ray outbursts reveal another neutron star's
surface.

The broader implication of the Chandra discovery is
that binary star systems with a neutron star orbiting a
normal star may be common in globular clusters.
Previously, and inexplicably, astronomers had never
seen more than one of these neutron star binaries in
any one globular clustera tight spherical region
that can contain a million stars or more.